PROJECT SUMMARY/ABSTRACT Transgender women (TW) are one of the most vulnerable populations for acquiring HIV infection, and the scant available data on transgender men (TM) suggests they are also at risk. TW have accounted for only 0.2% of all participants in bio-behavioral HIV prevention trials using pre-exposure prophylaxis (PrEP), and TM have typically not been included. TW and TM adolescents have received even less attention in PrEP trials. Although tenofovir disoproxil fumarate/emtricitabine (TDF/FTC), the FDA-approved PrEP drug, would not be expected to interact with cross-sex hormone therapy (cs-HT; estradiol and testosterone) based on known mechanisms and data from studies with hormonal contraceptives, there are no data that prove this. Given this lack of data, TW and TM youth on cs-HT have decreased PrEP uptake and adherence due to concerns that PrEP may reduce the effectiveness of cs-HT. To address these critical scientific gaps in PrEP safety for transgender youth and to plan for appropriate implementation we propose the following study in 3 integrated phases. In Phase 1, we will conduct a PK study exploring the interactions of cs-HT for both TW and TM on TDF/FTC. Simultaneously, in Phase 2, we will collect ethnographic data via focus groups and in-depth interviews to inform the development of a tailored gender-affirmative intervention to improve uptake and adherence to PrEP in transgender youth. In Phase 3, we will conduct a small demonstration trial of PrEP use in TW and TM youth, utilizing the ethnographically developed intervention to improve uptake and adherence, while also monitoring renal and bone safety outcomes. The project has the following specific aims: Aim 1: To evaluate the differential PK of TDF/FTC in a cohort of transgender youth on cs-HT by conducting a PK trial of daily TDF/FTC among 24 TW taking estradiol and 24 TM taking testosterone (ages 15-24 years) using video-based directly observed therapy (DOT) to insure daily adherence and maximize drug exposure. Aim 2: To develop a culturally, developmentally, and gender- affirmative intervention to increase uptake of and adherence to PrEP among TW and TM youth that is grounded in theory (Information-Motivation-Behavioral Skills Model of Behavior Change, Gender Affirmation, Empowerment Theory) and incorporates the PK data from Aim 1. We will conduct FGs with young TW (N=20- 30) and TM (N=20-30) and conduct IDIs with participants from the PK study (Total N=10-14). We will solicit continuous input and feedback from TW and TM on the project's Youth Advisory Board. Aim 3: To conduct a small randomized controlled trial within a PrEP demonstration project comparing the newly developed intervention with standard of care (SOC) in TW (N=50) and TM (N=50) ages 15-24 years.